The tendency of optical fibers to leak optical energy when bent has been known since the infancy of the technology. It is well known that light follows a straight path but can be guided to some extent by providing a path, even a curved path, of high refractive index material surrounded by material of lower refractive index. However, in practice that principle is limited, and optical fibers often have bends with a curvature that exceeds the ability of the light guide to contain the light.
Controlling transmission characteristics when bent is an issue in nearly every practical optical fiber design. The initial approach, and still a common approach, is to prevent or minimize physical bends in the optical fiber. While this can be largely achieved in long hauls by designing a robust cable, or in shorter hauls by installing the optical fibers in microducts, in all cases the optical fiber must be terminated at each end. Thus even under the most favorable conditions, certain bending can be encountered at the optical fiber terminals.
Controlling bend loss can also be addressed by the physical design of the optical fiber itself. Thus, ring features or trench features, or combinations thereof, are commonly found at the outside of the optical fiber refractive index profiles to control bend losses. See for example, U.S. Pat. Nos. 4,691,990 and 4,852,968, and U.S. patent application Ser. No. 12/583,212, filed Aug. 17, 2009, all incorporated herein by reference.
In a conventional graded index multimode fiber, high order modes suffer greater loss than low order modes when the fiber is bent. In bend-optimized multimode fiber designs (BOMMF), a trench is located at the outer edge of the graded index core to minimize the bend loss of high order modes. As is well known in this art, a trench refers to an annular region of depressed index located in, and part of, the cladding region.
In typical optical fiber designs in which a negative trench is used to aid in controlling bend loss the negative trench is spaced from the edge of the alpha profile of the core by a shoulder. Typically the shoulder is silica (refractive index delta of zero). The width of the shoulder is a design variable used in combination with other design parameters to modify fiber characteristics. While these designs produce exceptional bend loss performance improvements are still sought.